evaluation of a 3 Tesla whole-body RF coil and have been encouraged by the quality of images obtained. In order to obtain a quantitative evaluation of RFÂ ...
In-vivo Mapping of B1 Uniformity Produced by a Whole Body 3T RF Coil David ALSOP1, Ronald Dean WATKINS2, Robert GREENMAN1, John SCHENCK2, Robert LENKINSKI1
1Beth Israel Deaconess Medical Center, Harvard Medical School, Department of Radiology, Boston, MA USA; 2G.E. Corporate R & D Center, One Research Circle, Niskayuna, NY USA; Introduction It has been predicted that at high field strengths, the resistive and dielectric properties of tissues will lead to very nonuniform RF fields(1). This RF nonuniformity can reduce the utility of imaging at high field strengths both because the contrast becomes nonuniform and the sensitivity in some locations is compromised. We have begun the evaluation of a 3 Tesla whole-body RF coil and have been encouraged by the quality of images obtained. In order to obtain a quantitative evaluation of RF uniformity within the body, we implemented a sequence for mapping of B1 that could step through multiple RF amplitudes within one breathhold. The resulting images were analyzed to produced B1 uniformity maps across coronal and axial planes. Methods Scanning was performed in a GE Signa VHi 3 Tesla scanner equipped with a prototype highpass birdcage body coil. (56 cm diameter, 50 cm in length). Two male normal volunteers were imaged following an IRB approved human study protocol. Power deposition was continuously monitored and was maintained within FDA guidelines. B1 mapping was performed with a modified single-shot fast spin echo (SSFSE) sequence. A field of view of 48 cm, a slice thickness of 10 mm, and a matrix size of 128x96 were selected. The refocusing RF pulse train was tailored to reduced flip angles in order to minimize RF power deposition(2). Ten images were obtained with the excitation pulse amplitude stepped from near zero to approximately 120 degrees. 4 slices were obtained with a TR of 4s. Images were reconstructed preserving phase information. Tests in phantoms indicated that the signal amplitude deviated slightly from a pure sinusoid most likely due to slice profile effects. The in-vivo data were fit to the function empirically determined from the phantoms in order to produce maps of B1 uniformity. Results SSFSE images were of good quality and demonstrated signal modulations consistent with the expected sinusoidal pattern. The fitting routine was robust and produced reasonable values throughout most of the slices except in the thorax where the low signal of the lungs and the cardiac cycle motion degraded the maps.
Figure 1 An example coronal SSFSE image, left, and the calculated B1 map for that slice, right, are shown. Discussion These results suggest that acceptable RF uniformity for many studies can be achieved with a whole body coil at 3 Tesla. The increased B1 at the right and left sides as well as the enhancement of the effect when the arms are laid next to the body suggest the proximity to the coil is causing enhanced coupling through the sides. At high field strengths, the geometry of the dielectric tissue can perturb the uniformity of the field even if the coil currents are perfectly sinusoidal(3). Dielectric effects may also explain the surprising uniformity of the RF coil along its length. This uniformity exceeds both simulated and measured uniformity for non-dielectric phantoms in the short coil. This uniformity is particularly advantageous for large field of view coronal and sagittal imaging. Alteration of the current pattern or geometry of the RF coil might permit even greater uniformity at this field strength. References 1. PA Bottomley, ER Andrew, Phys. Med. Biol. 23:630-43 (1978) 2. DC Alsop, Magn Reson Med 37:176-84 (1997) 3. DC Alsop, Magn Reson Med 40:49-54 (1998)
B1 maps were characterized by excellent homogeneity along the axis of the coil. The greatest variation of B1 was along the right-left direction. B1 was highest near the right and left edges and decreased with distance to the center. B1 at the right and left edges exceeded 1.7 times the B1 near the center of the body. This spatial variation was more pronounced when the arms were laid along the side of the body than when the arms were raised above the head.
Proc. Intl. Soc. Mag. Reson. Med 9 (2001)
1094
Proc. Intl. Soc. Mag. Reson. Med 9 (2001)
1094
